This invention relates to a circuit for detecting interference signals received in a radio receiver and, more particularly, to an interference detection circuit suitable for use in a mobile radio communication system of multi-frequency access type.
In a wireless telephone system which is one type of mobile radio communication systems, as will be described in detail later, a plurality of radio substations are individually connected to subscriber lines, which are respectively coupled to corresponding radio telephone sets via radio channels. If different frequencies are allotted to respective radio channels, there will be no interference among these channels under normal conditions. In practice, however, to utilize the frequencies more effectively, the radio channels of the adjacent radio zones share the same frequency band. This causes these radio channels to interfere with each other. Such interference provides a beat in the received signals because there is a slight frequency difference in the radio channels of the same frequency. To avoid the beat and establish normal telephone services, the channel concerned should be switched to another free channel.
Such switching of the channel is conventionally carried out by manually operating the switch when a subscriber hears beat sounds. However, in order to confirm the beat sound, he has to listen to it for a certain duration of time. This is not only uncomfortable to the subscriber but also is detrimental to the system as the communication over the radio channel should be suspended for the time being.